As memory devices of all types have evolved, continuous strides have been made in improving their performance in a variety of respects. For example, the storage capacity of memory devices has continued to increase at geometric proportions. This increased capacity, coupled with the geometrically higher operating speeds of electronic systems containing memory devices, has made high memory device bandwidth ever more critical. One application in which memory devices, such as dynamic random access memory (“DRAM”) devices, require a higher bandwidth is their use as system memory in computer systems. As the operating speed of processors has increased, processors are able to read and write data at correspondingly higher speeds. Yet conventional DRAM devices often do not have the bandwidth to read and write data at these higher speeds, thereby slowing the performance of conventional computer systems. This problem is exacerbated by the trend toward multi-core processors and multiple processor computer systems. It is currently estimated that computer systems operating as high-end servers are idle as many as 3 out of every 4 clock cycles because of the limited data bandwidth of system memory devices. In fact, the limited bandwidth of DRAM devices operating as system memory can reduce the performance of computer systems to as low as 10% of the performance of which they would otherwise be capable.
Various attempts have been made to increase the data bandwidth of memory devices. For example, wider internal data buses have been used to transfer data to and from arrays with a higher bandwidth. However, doing so usually requires that write data be serialized and read data deserialized at the memory device interface. Another approach has been to simply scale up the size of memory devices or conversely shrink their feature sizes, but, for a variety of reasons, scaling has been incapable of keeping up with the geometric increase in the demand for higher data bandwidths. Proposals have also been made to stack several integrated circuit memory devices in the same package, but doing so threatens to create a large number of other problems that must be overcome.
One potential problem with increasing memory capacity to achieve a higher memory bandwidth is the higher likelihood that at least some of the memory cells will be defective. As is well-known in the art, memory devices typically have at least some memory cells that are defective, either at manufacture or after use. These defective memory devices are conventionally repaired by substituting redundant memory cells for the defective memory cells. Such repairs are normally accomplished by substituting a redundant row of memory cells for a row containing one or more defective memory cells or associated circuitry, or by substituting a redundant column of memory cells for a column containing one or more defective memory cells or associated circuitry. Yet vastly increasing memory capacity can make it more difficult to repair memory devices by substituting redundant memory cells for defective memory cells.
Therefore, a need exists for a method and apparatus to minimize problems and limitations caused by greatly increasing the data bandwidth of memory devices, such as the need to repair memory devices containing defective memory cells.